I. Field of the Disclosure
The technology of the disclosure relates generally to structures and methods for filtering radio frequency (RF) signals, and more specifically to cavity filters for filtering RF signals.
II. Background
Wireless computing devices have become common in contemporary society. These computing devices receive and/or transmit wireless signals, such as radiofrequency (RF) signals, and rely on microprocessors and other integrated circuits (ICs) for signal processing. In both mobile devices, like smart phones, and stationary computing devices, such as desktop computers, there is a general trend toward decreasing the size of such ICs. As device sizes decrease, the available space for individual components has also decreased. There is also a trend towards providing integrated circuits for mobile devices in a system-on-a-chip (SoC). An SoC is an integrated circuit (IC) that integrates components of a computer and other electronic systems into a single chip. The SoC may contain digital, analog, mixed-signal, and often radio-frequency functions all on a single chip substrate.
In many ICs including SoCs, RF filters are commonly used to pass and/or block specific frequencies or frequency bands in an RF signal or signals. For example, a signal of interest may be contained in a sixty (60) GigaHertz (GHz) band, but a device antenna may receive frequencies across a significantly larger portion of the RF spectrum. An appropriately configured RF filter can pass the band containing the signal of interest while effectively rejecting or blocking other signals and noise contained in frequencies above and below the desired band. It is generally desirable that an RF filter pass as much of the signal in the desired pass frequency band as possible while also blocking as much of the outside spectrum as possible.
Different types of RF filters have varying filtering quality levels, referred to as a ‘Q’ factor (Q), which is inversely proportional to the fractional bandwidth. While it is generally desired to employ RF filters in circuits that have the highest ‘Q’ factor, different types of RF filters also have different drawbacks, which may affect the type of RF filter employed. For example, conventional waveguide-type filters have high-Q factor, but are relatively large in size, and therefore unsuitable for many ICs that require smaller component sizes. On the other hand, conventional microstrip-based filters are compact and easily integrated into silicon layers of a semiconductor component, but microstrip-based filters have relatively low-Q factor with relatively high signal loss. Microstrip-based filters may also be relatively difficult to isolate from nearby components, thus resulting in undesired coupling and interference with those components.
Another type of RF filter with a high-Q factor is a cavity filter. A cavity filter employs a resonator cavity that is tuned to the desired frequencies or frequency bands and able to pass those bands with low insertion loss and with high isolation from nearby components. However, if the cavity filter is desired to be employed in a circuit in a small package or application, such as a mobile application, there must be room to provide a resonator cavity. Further, the dimensions for the resonator cavity are tied to the desired frequency bands and therefore present additional design challenges for mobile applications and other applications where space and component sizes are limited.